The present invention relates to a steel member for use under hot or warm conditions, particularly a hot- or warm-working die, and a method for producing it.
Dies for hot or warm forging are conventionally made mainly of hot-working tool steel such as SKD61, SKT4, etc. defined by JIS, and dies required to have higher durability are made of high-speed steel having better high-temperature strength such as SKD7, SKD8, etc. or steel improved therefrom.
Recent trend is generally that hot- or warm-working dies are surface-treated to impart high wear resistance and galling resistance to die surfaces while keeping toughness thereof, in response to the demand of higher precision and efficiency in working. The surface treatment applied to such dies is mainly a single-step nitriding treatment by an ion process, a salt bath process, a gas process, etc.
For instance, Japanese Patent Laid-Open No. 7-138733 discloses a method for providing a steel die with a heat cracking resistance and a plastic flow resistance by subjecting the die to an ion-nitriding treatment and then to high-frequency heating at a temperature up to 950.degree. C. to reduce an outermost, brittle white layer containing high-concentration nitrides and to make a nitrogen-diffused layer as deep as 3.0 mm. Also, Japanese Patent Laid-Open No. 57-54551 discloses a hot-working die which is iron-nitrided at a low temperature (350-450.degree. C.) to prevent galling while keeping the toughness of the die substrate. The effects of these methods are, however, limited to as small as 20 to 30% increase in a die life as compared with the conventional nitriding methods, suggesting that these methods are not regarded as providing remarkable improvement of the die life.
There is also a trend of near-net shaping which leads to worked articles with more complicated shapes. The near-net shaping makes larger the plastic flow of work materials during working and thus increase friction between the works and die surfaces, which results in making the temperature of the dies higher than their transformation temperatures of 700-900.degree. C. and thus accelerating the softening of die surfaces by friction heat. As a result, the dies lose their inherent properties, resulting in poor high-temperature properties and accelerated damage of the dies.
When a single-step nitriding treatment such as an ion-nitriding treatment which is the most prevalent surface treatment for steel members at present is carried out on dies, the resultant nitrides are partially decomposed by overheat, failing to provide sufficient effects.
Other than the single-step nitriding treatment, Japanese Patent Laid-Open No. 4-228557 proposes a method and an apparatus for gas-sulfonitriding cold-slidable members operable in oils such as pistons, cylinders, etc. of hydraulic pumps, motors, etc. of construction machines in order to improve their properties of keeping lubricating oils. This method forms a surface layer comprising ferric sulfide (FeS.sub.2) highly capable of keeping a lubricant as an outermost surface layer on a steel member by a secondary heat treatment at 200-350.degree. C.
Japanese Patent Laid-Open No. 60-39155 proposes the formation, on a surface of an iron article, of a first layer mainly composed of ferric sulfide (FeS.sub.2) and a second layer of iron nitride (Fe.sub.4 N) by bringing a decomposed gas of ammonium sulfide and an ammonia gas into contact with the iron article while heating. However, the resultant layers are likely to be porous, providing starting points of heat cracking and propagating paths thereof. Accordingly, dies produced by this technology cannot suitably be used for hot-working at a temperature of 600.degree. C. or higher and a high pressure.
Katagiri et al. reported in The Japan Association of Metallurgy, Vol. 51, No. 10 (1987), pp. 930-934 the sulfonitriding treatment for forming an outermost porous surface layer of ferrous sulfide (FeS) and an underlying surface layer containing iron oxide (Fe.sub.3 O.sub.4) on a steel material in 150 ppm of a hydrogen sulfide gas and 75% of an ammonia gas at 580.degree. C. for 1-6 hours, by using a colorless ammonium sulfide solution. However, since this method uses a colorless ammonium sulfide solution as a starting material, a weight ratio of sulfur to oxygen (S/O) in the resulting surface layer is less than 0.5, failing to sufficiently lower a friction coefficient between the die surface and the work. Also, since there are likely to be starting points of heat cracking and propagating paths thereof in the porous layers, dies produced by the method of Katagiri are not suitable for plastic working at high temperature and pressure.
Momijizawa reported in Heat Treatment, Vol. 36, No. 6 (1996), pp. 383-387 the formation of a sulfurized layer made of FeS or Fe.sub.1-x S having solid lubrication in a thickness of 3-5 .mu.m in addition to a nitride layer (on the substrate side) to improve galling resistance and wear resistance at room temperature (20.degree. C.), by a gas-nitriding cycle comprising various treatment cycles using gases supplied from an H.sub.2 S/N.sub.2 bottle, a pure N.sub.2 bottle, a pure NH.sub.3 bottle and a CO.sub.2 bottle. The resultant surface layer of steel has a nitride layer and a black sulfurized layer with solid lubrication on this order from a steel substrate. It is reported that since sulfur substantially does not form a solid solution with (.alpha.-Fe unlike nitrogen, the sulfurized layer of FeS or Fe.sub.1-x S is limited on a surface of the steel, not being diffused inside it. However, if dies subjected to such a treatment are used for high-temperature working, ferric sulfide (FeS.sub.2) and ferrous sulfide (FeS or Fe.sub.1-x S) easily peel off from the nitride layer due to the difference in thermal expansion coefficient therebetween. Thus, the dies treated by this method cannot be used as hot- or warm-working dies.
In addition, Japanese Patent Publication No. 7-42566 proposes the formation of iron oxide (Fe.sub.3 O.sub.4) on iron articles made of soft steel or cast iron such as bolts, nuts, etc. to prevent corrosion in their portions buried in earth and also to improve corrosion resistance and appearance in their portions exposed in the air.
In general, the damage of a die takes place in a manner as described below during the plastic working at a high temperature. A surface of the die is subjected to thermal shock by contact with the work as follows: The plastic working is carries out at a high temperature such that the heated work plastically flows along a working surface of the die while being pressed onto the die surface, thereby generating friction heat and plastic deformation heat. During this plastic working process, the die surface is subjected to quick thermal expansion by rapid temperature increase. After completion of the plastic working, the worked article is removed from the die, resulting in the shrinkage of the die surface by cooling.
As a result of repeated plastic working as described above, the die surface is not only subjected to thermal fatigue by expansion and shrinkage, but also the die surface softened by heat has a decreased resistance to stress generated by plastic working and expansion and shrinkage, so that heat cracking and plastic flow are more likely on the die surface. Thus, damage such as wear proceeds on the die surface. In this case, galling would be likely to take place if the die surface is in direct contact with the work. The generation of galling makes easier thermal conduction from the work to the die surface, accelerating the damage of the die.
Accordingly, a lubricant or a parting agent is applied onto the die surface every cycle in an actual operation, so that the die surface and the work are not in direct contact with each other by the lubricant or the parting agent present in a film state between them. However, the die surface heated to a high temperature is rapidly cooled by applying the above agent, resulting in larger shrinkage in a unit time.
As described above, a surface layer formed on a steel material by these sulfonitriding methods is likely to provide starting points of heat cracking and propagating paths thereof due to its porosity, when the surface-treated steel material is used as a die for plastic working at a high temperature. Also, ferric sulfide (FeS.sub.2) and ferrous sulfide (FeS or Fe.sub.1-x S) easily peel off from an iron nitride layer to which these sulfides are adjacent, due to the difference in thermal expansion coefficient therebetween. Thus, the dies treated by these sulfonitriding methods cannot be used as hot- or warm-working dies.